Investors are in a game of wait-and-see with the Federal Reserve. On Monday, they sent stocks higher as they guessed that the Fed would continue trying to prop up the economy.

The major stock indexes all rose about 1 percent in early trading and stayed there for most of the day before dipping slightly in the afternoon. The Standard & Poor’s 500-stock index rose 12.31 points, or 0.8 percent, to 1,639.04. It had been up as much as 20 points.

The market’s gains were broad. Telecommunications was the only one of the 10 industry sectors in the S.& P. 500 to post a loss. Netflix did better than any other stock in the S.& P. 500 after announcing that it would run original TV series from DreamWorks Animation.

There were few big company announcements or economic reports, and trading was light. Investors will have to keep guessing about the Fed’s future actions until Wednesday, when the chairman, Ben S. Bernanke, holds a news conference at the end of a two-day policy meeting.

Investors sent stocks up Monday because they think Fed policy makers will determine that the economy is not recovering fast enough. A still-weak economy would influence the Fed to continue its programs intended to stimulate the economy: keeping interest rates low to encourage borrowing, and buying bonds to push investors into stocks.

Doug Lockwood, branch president of Hefty Wealth Partners in Auburn, Ind., said it was not rational for the stock market to regard bad news as good, and to be yanked back and forth more by the actions of a central bank than the underlying fundamentals of the economy.

The market has been in flux since May 22, when Mr. Bernanke said that the Fed would consider pulling back on its bond-buying program if measures of the economy, especially hiring, improve. The comment, made in response to a question from the Joint Economic Committee in Congress, was not expected. In the 17 trading days since then, the Dow Jones industrial average has swung by triple digits 11 times.

On Monday, the Dow rose 109.67 points, or 0.7 percent, to 15,179.85. The Nasdaq composite rose 28.58, or 0.8 percent, to 3,452.13.

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The price of crude oil rose throughout the day but ended 4 cents lower at $98.03 a barrel in New York. Gold edged down $4.50 to $1,383.10 an ounce.

In the market for government bonds, the benchmark 10-year Treasury note fell 13/32 to 96 7/32, bringing the yield up to 2.18 percent from 2.13 percent late Friday.

Jim McDonald, chief investment strategist at Northern Trust in Chicago, said Mr. Bernanke would seek to “walk back” on some of his previous comments, and reassure investors that the Fed will not pull back on stimulus until it is sure the economy is ready. The surprise factor, more than the substance of Mr. Bernanke’s comments, might have been what unnerved investors, McDonald said.

The fact that Mr. Bernanke is now expected to regard the economy as still weak enough to need stimulus stems from a jobs report and low inflation since his testimony, analysts said.

This month, the government reported that the United States added 175,000 jobs in May — not enough to cut into the unemployment rate. And on Friday, the government said that a crucial measure of inflation — the producer price index, which measures wholesale prices — rose just 0.1 percent after stripping out the volatile costs of food and gas. That is important because the Fed knows that its stimulus measures can stoke inflation; if inflation is low, the central bank has more flexibility to keep pumping money into the economy.

Two measures of economic data released on Monday were positive, though both are considered less important gauges of the economy. A report on manufacturing in New York State showed a pickup, and a survey of American home builders said they were more optimistic about sales than they had been in seven years.

A version of this article appears in print on June 18, 2013, on Page B8 of the New York edition with the headline: Markets Rise on Thought That Fed Will Continue Stimulus. Order Reprints|Today's Paper|Subscribe